The tendency of iron or steel surfaces to corrode is well known in the industry. Very often, a metallic coating of nickel or zinc, or mixtures thereof, is applied to the surfaces to protect them from further corrosion. Zinc and nickel have been electroplated onto iron and steel substrates from various plating baths, preferably from acid plating baths, for protection of the surface for various uses in industry.
One such electroplating apparatus is described in U.S. Pat. No. 4,840,712 to Steinbicker et al., which is incorporated herein by reference, and discloses a pair of conductor rolls which direct a steel strip between a pair of plating anodes in a plating bath. The electroplating process utilizes a zinc sulfate electroplating solution as well as sulfuric acid rinsing water, so that the stainless steel conductor rolls are subjected to electrochemical corrosion and mechanical wear. The Steinbicker patent discloses a method for improving wear life of the conductor rolls by employing hydrogen peroxide as a passivating film formation agent within the rinse solution.
In recent years, the electroplating industry has experienced a steep rise in the cost of some of the metals used in coating iron and steel substrates. This is particularly true in the case of nickel, the price of which has fluctuated dramatically over the past few years. Nickel prices jumped from $2.37/lb. in January 1987 to over $7.00/lb. in April 1988. These large cost changes suggest some means for recovering the metal is required in order to minimize expense.
In addition, government regulations on discharge of effluents into the environment have increased dramatically. The electroplating industry has been especially hard hit by EPA disposal regulations, due to the solutions and wastes which arise during the electroplating process. Electroplating sludge is currently classified under EPA regulations as a "hazardous" waste, thereby increasing the cost and difficulty of its disposal. As a result, a steady increase in reprocessing and recovery of nickel, zinc and other plating metals from rinse solutions and plating baths has been experienced.
One method of recovering nickel and zinc from plating baths and rinse solutions includes treatment with an ion exchange resin to selectively adsorb the nickel or zinc cations onto the resin. A subsequent acid washing step will remove the recovered metals from the resin. Other available methods include electrodialysis and reverse osmosis technology. The use of ion exchange resin and other technology to remove useful metals from electroplating baths and rinse water is disclosed in U.S. Pat. No. 4,783,249 to Fishman, U.S. Pat. No. 4,009,101 to Hayashi, U.S. Pat. No. 3,761,381 to Yagishita, U.S. Pat. No. 3,681,210 to Zievers et al. and in U.S. Pat. No. 3,630,892 to Hirs et al.
Direct recovery of waste metals is most efficient from streams that are consistent in both flow and composition. For example, the conductor roll rinse and the rinse after plating streams are generally clean and constant in terms of flow and metal concentration. Ninety-five percent of the nickel and zinc contained in these wastewaters can be recovered by the ion exchange bed process.
"Other" waste streams are produced during the plating process, but these streams are too dirty and variable in flow and concentration to lend themselves to conventional ion bed separation. These dirty waste streams include effluent from the ion exchange units, end of run washdown flows and overflows from sumps, drains and spills.
Applicants have developed a method for direct recovery of the bulk of metals from the streams that are most consistent in flow and composition as well as reclamation of those metals that escape this primary metal recovery. Direct recovery of zinc and nickel is made from separately collected, relatively clean flows via two reciprocating flow, short-bed ion exchange units. The effluent from this process is then blended with other flows, such as drains, washdowns and spills, so that process upsets are minimized. These blended, variable streams are neutralized and precipitated in a two-stage process. The resultant slurry is filtered to yield a filter cake containing nickel and/or zinc which can then be further processed for reuse.
Although the use of an ion exchange resin has greatly increased the efficiency of recovering zinc and nickel from the plating waste streams, hydrogen peroxide added to the rinse solution to minimize conductor roll corrosion has an adverse effect on commercial cation exchange resin beads. Even minute quantities of hydrogen peroxide in the waste streams will oxidize the internal bonds that maintain the resin shape, thereby causing swelling and eventual depolymerization of the resin. Resin swelling causes a gradual increase in pressure drop across the ion exchange bed, and the eventual need for replacement. Hydrogen peroxide has a similar effect on electrodialysis and reverse osmosis membranes.
The present invention also discloses a method of removing the hydrogen peroxide from the conductor roll rinse solution during processing of the rinse water and plating bath solutions so that metal recovery of zinc and nickel may continue unimpeded. In the process, the conductor roll rinse, which contains hydrogen peroxide, is treated by contact with activated carbon to catalytically destroy the hydrogen peroxide before it can attack the ion exchange resin. The decomposition of the hydrogen peroxide occurs rapidly and effectively upon contact with the high surface area activated carbon bed.